Perhaps the most familiar parts of a computer are the central processing unit
and the main memory. The main memory is the same as the "computer's
memory," also commonly referred to as system memory, or RAM. In this chapter
we examine memory chips and how they're packaged. In Chapter 5, "Processors
and Chipsets," we then look at different types of processors.

Memory is only a temporary place to store information until a device can get
to it. Essentially, the CPU uses memory to move program instructions and data
in and out of that temporary storage area. A typical instruction might be a
request to store a data bit somewhere. Another instruction might be to retrieve
that bit from a particular placean address. Data might be a number,
a letter, or any other bit of information. Remember that a data bit is also
a small charge of electricity.

Data becomes information when it takes on context (surrounding circumstances).
76 means nothing on its own, other than the fact that it's a number. Surround
that number with context: "Tomorrow, the temperature is expected to reach
76," and it becomes information. RAM is like a holding tank for data on
its way to becoming information.

Storage is any location where information can be placed and retained
for some amount of time. Computer memory is temporary storage, in that it generally
requires the presence of electrical current. Volatile memory can hold
information only when a normal electrical current is present. Nonvolatile
memory can hold information in the absence of an electrical current. System
memory is, for the most part, volatile.

Compact memory cards, smart memory cards, and memory sticks used in digital
photography are examples of nonvolatile memory. Although some amount of current
is necessary to change information, that information then remains stored even
when there is no further current. Remember that volatile memory requires a continuing
supply of current.

NOTE

Volatile, from the Latin "to fly," means that
information "flies away" when there's no electricity to keep
it in place. Television reporters often refer to an explosive situation as
a volatile situation, meaning that it could change at any second. Volatile
memory will lose all of its data when electricity is removed. Nonvolatile
memory will maintain its data even without electricity.

Floppy disks, fixed disks, optical disks, and card media are all nonvolatile.
However, although disks retain information without electrical current, we
refer to them as permanent storage, not "memory." The terms volatile
and nonvolatile are generally assigned to memory chips.

Permanence is a relative word. A burst of static electricity can completely
wipe out any information on a magnetic storage device or in a memory chip. Optical
disks store data in structural changes to the media, and so even ESD or close
proximity to a magnet (electromagnetic interference) rarely affect that information.
Chapter 6, "Basic Electronics," examines both ESD and EMI.

Conceptual Overview

Computer memory is fairly easy to remember when you've grasped the basic
concepts. In a nutshell, a CPU uses transistors to handle bits of data. These
transistors are grouped together into registers, making for small storage places
inside the processor housing (the chip die). At some point, either the registers
fill up or the instructions are completed. The CPU then works together with a
memory controller to move data bits out to memory cells. Memory cells are
typically capacitors that form small storage places on a memory chip. Both
processor registers and memory cells have addresses. Every time a bit of data
goes somewhere, it crosses a bus of some kind. That's it; now go pass the
exam!

All right, so it's a bitso to speakmore complicated than
that. Most memory began as dynamic random access memory(DRAM).
The main engineering changes that have taken place have all been attempts to
find ways of either speeding up the memory to match CPU speeds, or to speed up
the CPU to match memory speeds. The rest of memory technology relates to moving
bits of information across buses more quickly.

TIP

When we refer to speeding up memory, we usually mean increasing the speed of
the memory chips, increasing the clock speed of associated buses, or handling
larger pieces of data.

To understand memory addresses, you should first understand a grid or
matrix. We're therefore going to use Table 3.1 to a slightly
different fashion, making it into a sort of "mind map." If you can see
the way the overall types of memory break down on a grid, then perhaps
they'll be easier to remember.

NOTE

A matrix is nothing more than an arrangement of columns and rows, like a
spreadsheet or an Etch-a-Sketch. Columns go up and down across the page, and
rows go left and right across the page. Remember the word "page."
Column addresses are at the top; row addresses are along the side.

Cells going left to right (horizontally) in a row have an X coordinate. Cells
going up and down (vertically) in a column have a Y coordinate. The direction of
rows is called the X axis, and columns are called the Y axis. Combining X and Y
coordinates gives us an address in the grid, like a cell address in a
spreadsheet. A memory page is a range (group) of cell addresses within a row.